1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method and apparatus for mitigating interference in a wireless communication system supporting heterogeneous networks.
2. Discussion of the Related Art
With reference to FIG. 1, a frame structure for a wireless communication system will first be described. FIG. 1 illustrates a frame structure for a Long Term Evolution (LTE) system. Referring to FIG. 1, a frame includes 10 subframes, each subframe having 2 slots. Time taken to transmit one subframe is called a Transmission Time Interval (TTI). For example, one subframe may be 1 ms long and one slot may be 0.5 ms long.
One slot includes a plurality of Orthogonal Frequency Division Multiplexing (OFDM) symbols. An OFDM symbol may also be called a Single Carrier-Frequency Division Multiple Access (SC-FDMA) symbol or a symbol period.
One slot includes 7 or 6 OFDM symbols depending on the length of a Cyclic Prefix (CP). In the LTE system, a normal CP and an extended CP are defined. One slot includes 7 OFDM symbols in the case of a normal CP, whereas one slot includes 6 OFDM symbols in the case of an extended CP.
FIG. 2 illustrates the resource structure of one downlink slot. Herein, a slot includes 7 OFDM symbols. A Resource Element (RE) is a resource area occupying one OFDM symbol by one subcarrier, and a Resource Block (RB) is a resource area occupying a plurality of OFDM symbols by a plurality of subcarriers. For instance, an RB may include 7 OFDM symbols in time by 12 subcarriers in frequency. The number of RBs per slot may be determined according to a downlink bandwidth.
FIG. 3 illustrates a downlink subframe structure. Referring to FIG. 3, up to three first OFDM symbols in the first slot of a subframe correspond to a control region to which a control channel is allocated. The other OFDM symbols correspond to a data region to which a Physical Downlink Shared Channel (PSCH) is allocated.
In the 3rd Generation Partnership Project (3GPP) LTE system, downlink control channels include a Physical Control Format Indicator Channel (PCFICH), a Physical Downlink Control Channel (PDCCH), and a Physical Hybrid automatic repeat request Indicator Channel (PHICH).
The PCFICH is transmitted in the first OFDM symbol of a subframe, carrying information about the number of OFDM symbols used for transmission of control channels in the subframe. The PHICH delivers an ACKnowledgment/Negative ACKnowledgment (ACK/NACK) signal as a response to uplink transmission. Control information transmitted on the PDCCH is called Downlink Control Information (DCI). The DCI includes uplink scheduling information, downlink scheduling information, or an uplink transmit power control command for a User Equipment (UE).
Now a description will be given of carrier aggregation in an LTE-Advanced (LTE-A) system with reference to FIG. 4.
FIG. 4(a) illustrates a single carrier structure and FIG. 4(b) illustrates a multi-carrier structure.
As there has been an increasing demand for high data rates, studies have been conducted on carrier aggregation. Referring to FIG. 4(b), a UE may simultaneously receive downlink signals on multiple carriers. However, even when a Base Station (BS) manages N downlink Component Carriers (CCs), the BS may allocate M downlink CCs to a UE so that the UE monitors signals on the M downlink CCs. Herein, M is equal to or less than N. The BS may also allocate L CCs as primary CCs to the UE so that the UE monitors signals on the L CCs with priority. L is equal to or less than M.
If UEs of heterogeneous networks communicate with a BS in the same frequency band in a heterogeneous network environment, interference may occur. For example, in the case where a femtocell BS is installed within the coverage of a macro BS, if UE A near to a femtocell managed by the femtocell BS communicates with the femtocell BS and UE B near to UE A communicates with the macro BS in the same frequency band as UE A, a signal that UE B transmits to the macro BS interferes with UE A. Especially, considering that the Modulation and Coding Scheme (MCS) level of a Physical Uplink Control Channel (PUCCH) is not changed much, it is difficult to decrease the power of a control channel transmitted by UE A. Accordingly, the PUCCH transmitted by UE A may interfere with the femtocell significantly.